FireNet Community
FIRE SERVICE AND GENERAL FIRE SAFETY TOPICS => Service Training OR Recruitment => Topic started by: jasper on September 22, 2011, 01:18:13 PM
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just wondered if any of you can provide a basic time to fill calc for a pair of portal frame storage buildings (did this 10+ years ago but forgot now) pm if interested
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Time to fill with what?
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smoke? nm
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Try the link below
http://www.hugedrive.com/published/WG/show.php?q=Q0FMQ1NOU1RVRkY=-b65b0aa8
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I'm sure your model is a reasonable approximation, Bleve, but I notice that it assumes that the fire is burning at one rate of heat output from the outset. Users should be aware that this is likely to lead to conservative results. In many cases such results will be more than adequate but there are more realistic models available. Mind you, I'm not claiming that any of them give accurate results.
I also notice, Bleve, that the formula for tau m seems to be the ratio between the mass of ambient air in the space and the mass flow rate of smoke per second. But the figure used for the mass flow rate of smoke is correct only when the smoke layer has zero thickness (because H^5/3 is used). Is this correct?
Also, any users of models such as these should be aware that they should not be used for spaces larger than about 1,000 sq m in area (that would be stretching the limits of feasible extrapolation) and they probably start to lose reliability in spaces smaller than this - maybe at around 500 sq m.
Also, why is this under 'service training or recruitment'?
Stu
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Also, I just noticed this was for portal framed buildings. Well, these tend to have sloping ceilings and the models tend to assume flat ceilings, so don't forget to make the appropriate adjustments to the figures. These adjustments can become rather complex, it's not just a case of calculating the 'flat roof equivalent' height by converting the volume of the space under the sloping ceiling to a cuboid with the same area as the space, you also have to consider the changes to the height of rise of the plume.
Also, portal framed buildings are often quite large and you may fall foul of the area limitation I mentioned above.
Stu
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Very true, but the OP asked for basic calculation.
I thought i would steer away from dimensionless heat and time values or risk annoying Civvy.
Generally the slope of a portal frame is not that great and the average height between gutter and ridge can be used.
In reality I would go with CFD.
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Indeed